Python script executed from Dockerfile doesn't write HTML file

Question

I am trying to alter some .html files whilst building a docker image. I have created a add_style.py which alters the html to add a line of css. First let me show you the Dockerfile :

FROM golang:1.15.2-buster as ClaatSetup
RUN CGO_ENABLED=0 go get github.com/googlecodelabs/tools/claat

FROM alpine:3.10 as ClaatExporter
WORKDIR /app
COPY --from=ClaatSetup /go/bin/claat /claat
COPY docs/ input/
RUN /claat export -o output input/**/*.md


FROM alpine:3.10 as AppCompiler
RUN apk add --update git nodejs npm make python gcc g++ && \
    npm install -g gulp-cli

WORKDIR /app

# against caching when there is a new commit
ADD "<..>" /tmp/devalidateCache
RUN git clone <..>
WORKDIR /app/codelabs-site

# Install dependencies
RUN npm install && npm install gulp

# Copy exported codelabs from previous stage
COPY --from=ClaatExporter /app/output codelabs/

# Build everything
RUN gulp dist --codelabs-dir=codelabs

# Replace symlink in with actual content (see below for description)
WORKDIR /app/codelabs-site/dist
RUN rm codelabs
COPY --from=ClaatExporter /app/output codelabs/



FROM caddy:alpine as Deployment
WORKDIR /app

COPY --from=AppCompiler /app/codelabs-site/dist/ .

#injecting of css to widen codelabs
RUN apk add --no-cache python3 py3-pip
RUN pip3 install bs4
COPY add_style.py codelabs/add_style.py
RUN python3 codelabs/add_style.py

EXPOSE 80
EXPOSE 443
CMD ["caddy", "file-server"]

Then the add_style.py itself:

import os
from bs4 import BeautifulSoup, Doctype

for root, dirnames, filenames in os.walk(os.getcwd()):
    for filename in filenames:
        if filename.endswith('index.html'):
            fname = os.path.join(root, filename)
            print('Filename: {}'.format(fname))
            with open(fname, 'r+', encoding = 'utf-8') as handle:
                soup = BeautifulSoup(handle.read(), 'html.parser')
                head = soup.head
                head.append(soup.new_tag('style', type='text/css'))
                head.style.append('.instructions {max-width: 1000px;} #fabs {max-width: 1200px;}')
                   
                #write altered html
                handle.write(str(soup.prettify()))
                handle.flush()

The latter script runs perfect, no errors. Also if you output soup.prettify() it shows the altered html with added CSS. Now if i read the file again in the same script it shows the old version.. I tried executing this script at different stages of the docker build but it just doesn't write the file. No errors are shown..

Answer 1

As by @Armando's comment. The solution is to add seek and truncate:

   #write altered html
   handle.seek(0)
   handle.write(str(soup.prettify()))
   handle.truncate()
   handle.flush()